In a time division, multiple access (TDMA) radio communication system, communication channels are provided by dividing the frequency spectrum and the time base into periodically repeated time frames and time slots at preassigned frequencies. Two stations in such a system communicate by transmitting and receiving on a specified frequency during specified time slots.
Such systems commonly include multiple fixed base stations configured for radio communication with one or more mobile stations, or handsets. The base stations are hard wired to a controller and to the public switched telephone network (PSTN) for completing telephone calls between a handset and the PSTN. Examples of such systems include cordless telephone systems according the Digital European Cordless Telecommunication (DECT) standard and the Personal Handyphone System (PHS) in Japan, and digital cellular telephone systems.
For accurate, efficient communications in such systems, the base stations must be synchronized to a common time base. If timing is not synchronized, the timing at which a signal is transmitted from one base station overlaps the signal transmission timing for another base station. This creates errors and inefficiency. A handset operating in the system synchronizes to the base station with which it is communicating. If base stations are unsynchronized, interference occurs in the communication between the handset and its base station. Also, hand-off of communication between the handset and a first base station to a second base station may not be possible. All base stations should be synchronized to a master clock for the system, kept, for example, at the controller. Synchronization may occur at the time of system set-up or when a new base station is installed or as part of routine system maintenance.
For synchronizing timing of the base stations, conventional systems provide a hard-wired synchronization line which couples each base station in the system to the master clock at the controller. Each base station establishes its timing in response to timing signals on the synchronization line. Handsets in the region near the base station synchronize to the timing established at the base station.
Use of the synchronization line introduces error in synchronization between base stations. The synchronization line adds a propagation delay to the synchronizing signals from the controller to the respective base stations. The length of the synchronization line between a base station and the controller varies from each base station. The propagation delay is proportional to the length of the synchronization line. Thus, the propagation delay for each base station is different, less for a base station near the controller and more for a base station remote from the controller. Each system specifies a maximum synchronization tolerance, or the maximum timing difference between the base station and a handset. For example, in a DECT system, the maximum synchronization tolerance is 2 .mu.sec. The propagation delays to remote base stations may cause such remote base stations to be outside the system synchronization tolerance and thus be unsynchronized. Propagation delays can only be matched by trial-and-error measurement of synchronization line lengths.
As an alternative to the synchronization line, other conventional systems transmit a synchronization signal from the controller to all base stations in the system using radio frequency communication. Each base station receives the signal and synchronizes its timing to the synchronization signal. This eliminates inaccuracies due to varying propagation delays in the hard-wired synchronization line.
However, such a system requires a controller with the ability to transmit to physically remote base stations. It may be expensive, impractical, physically impossible or not allowed by power allocation requirements, to provide a transmitter with sufficient transmit power for transmitting the synchronization signal to all base stations in the system.
Accordingly, there is a need in the art for an improved method for synchronizing base stations in a communication system which overcomes these limitations in conventional systems.